1. Field of the invention
This invention, relating to iron-based permanent magnets and alloy powders for iron-based bonded magnets and their fabrication, used for obtaining suitable iron-based bonded magnets for all kinds of motors, actuators and magnetic circuits for magnetic sensors, as well as magnetic rolls and speakers, regards iron-based permanent magnets and their fabrication which yield isotropic iron-based bonded magnets having a residual magnetic flux density Br greater than 5 kG unobtainable from hard ferrite magnets. These are produced by quenching an (Fe,Co)--Cr--B--R molten alloy or a (Fe,Co)--Cr--B--R--M (M=Al,Si,S,Ni,Cu,Zn,Ga,Ag,Pt,Au,Pb) molten alloy, with small amounts of rare earth elements added, using either, a melt-quenching process utilizing a rotating roll, splat quenching, a gas atomizing method, or a combination of these methods, to obtain an essentially amorphous structure or a structure containing small amounts of microcrystals dispersed within an amorphous matrix, to yield an iron-based permanent magnet consisting of microcrystal clusters where both a soft magnetic phase consisting of a ferromagnetic alloy whose main components are .alpha.-Fe and iron-based phases, and a hard magnetic phase having a Nd.sub.2 Fe.sub.14 B-type crystal structure coexist after applying a particular heat treatment. This is then ground to obtain an alloy powder for bonded magnets, which is then combined with a resin.
2. Description of the Prior Art
Permanent magnets used in stepping motors, power motors and actuators utilized in home electronic goods and electric goods in general are mainly limited to hard ferrites, which have various problems such as, demagnetization at low temperatures with the fall of iHc, the ease of formation of defects, cracks and a lowering of mechanical strength due to the quality of the a ceramic material, and the difficulty to fabricate complicated forms. These days, along with the miniaturization of home electronics and OA equipment, small, light-weight magnetic materials to be used in these products are being sought. As for motor vehicles, as much effort is being made towards saving money and resources by making vehicles light-weighted, even more small, light-weight electrical components for motor vehicles are being sought.
As such, efforts are being made to make the efficiency versus weight ratio of magnetic materials as large as possible and, for example, permanent magnets with a residual flux density Br in the range 5.about.7 kG are thought to be most suitable.
In conventional motors, for Br to be above 8 kG, it is necessary to increase the cross section of the iron plate of the rotor or stator which forms the magnetic path, introducing an associated increase in weight. Further, with the miniaturization of magnets used in magnetic rolls and speakers, an increase in Br is being sought as present hard ferrite magnets cannot give more than 5 kG Br.
For example, for a Nd--Fe--B-type bonded magnet to satisfy such magnetic characteristics, 10.about.15 at % of Nd needs to be included making their cost incredibly high compared to hard ferrite magnets. Production of Nd requires many metal separation and reduction processes which in turn needs large scale equipment. As well as this, for 90% magnetization, a magnetic field of close to 20 kOe is required and there are problems with the magnetization characteristics such as being unable to achieve complicated multipole magnetization such that the pitch between the magnetic poles is less than 1.6 mm.
At present, there are no permanent magnet materials with magnetization characteristics such that Br is 5.about.7 kG which can be mass produced cheaply.
Recently, a Nd--Fe--B-type magnet has been proposed whose main component is an Fe.sub.3 B-type compound with a composition close to Nd.sub.4 Fe.sub.77 B.sub.19 (at %) (R. Coehoorn et al., J. de Phys., C8, 1988, 669.about.670). This permanent magnet has a semi-stable structure with a crystal cluster structure in which a soft magnetic Fe.sub.3 B phase and a hard magnetic Nd.sub.2 Fe.sub.14 B phase coexist. However, it is insufficient as a rare earth magnetic material with a low iHc in the range 2 kOe.about.3 kOe, and is unsuitable for industrial use.
Much research is being published, however, on adding additional elements to magnetic materials with Fe.sub.3 B-type compounds as their main phase and creating multi-component systems, with the aim to improve their functionality. One such example is to add rare earth elements other than Nd, such as Dy and Tb, which should improve iHc but, apart from the problem of rising material costs from the addition of expensive elements, there is also the problem that the magnetic moment of the added rare earth elements combines antiparallel to the magnetic moment of Nd or Fe, leading to a degradation of the magnetic field and the squareness of the demagnetization curve (R. Coehoorn, J. Magn. Magn. Mat., 83 (1990) 228.about.230).
In other work (Shen Bao-gen et al., J. Magn. Magn. Mat., 89 (1991) 335.about.340), the temperature dependence of iHc was improved by raising the Curie temperature by replacing some Fe with Co, but this also caused a fall in Br on the addition of Co.
In each case, for Nd--Fe--B-type magnets whose main phase is an Fe.sub.3 B-type compound, it is possible to create hard magnetic materials with a heat treatment after amorphizing by quenching, but their iHc is low and the cost performance of using them instead of hard ferrite magnets is unfavourable. This incapability of providing a high-enough iHc is caused by a large grain size of the soft magnetic phase, typically 50 nm, which is not small enough to effectively prevent magnetization rotation in the soft magnetic phase from occurring under of a demagnetization field.